Antimatter, the mysterious mirror-stuff of the universe, is hard to make and harder to study. A laser that literally chills it out could change all that.
ADAM ROGERS, wired.com
March 31, 2021
THE THING ABOUT antimatter is that there just really isn’t very much of it at all. No one knows why. And making the stuff from scratch is like trying to win a GBBO showstopper. (The theme is “antiprotons.”) Plus, plain-vanilla matter and oppositely charged antimatter annihilate each other if they touch. Very finicky. So the real thing about antimatter is that physicists don’t know very much about it.
They have a good theory, though. Actually it’s the theory, the “standard model” that describes how subatomic particles are supposed to behave. Antimatter is supposed to do everything that matter does, only backwards-and-in-high-heels and looks-the-same-except-with-a-goatee. (More formally this is called “CPT symmetry,” as in charge-parity-time, which basically says that if you swap matter for antimatter and time reversed, the new universe would be the same as the current one.) It’s a theory; it needs testing, which is hard—see above. But it’s about to get a lot easier. A big group of scientists centered at CERN, the Swiss particle physics lab, was already the best in the world at making antihydrogen, the antimatter version of hydrogen. Today they published results in the journal Nature showing that they could freeze that stuff down to just fractions of a degree Kelvin—very, very cold. Cold atoms (and antiatoms) are sloooooooow, which makes them much easier to study. The secret to getting antimatter to chill out? Pew pew.
One well-understood way to get atoms to cool off is to slow them down—by shooting them with a laser. This makes more sense than you’d think. Motion, kinetic energy, is also heat. Lasers are made of light, and light is made of subatomic particles called photons. Photons, the wee-est little packets of electromagnetic energy, have momentum but no mass, juice but no oomph. When a photon with the right amount of energy—or the right wavelength, depending on how you want to think about it—hits an atom, that atom absorbs the photon, gains some energy, and then re-emits it. In the process, the atom literally recoils, bounces back a bit.
Now, those atoms are moving around, like in a cloud of gas. That means the actual wavelength of light that’ll do that trick is a little different for the ones moving toward the laser versus the ones moving away, thanks to the Doppler effect. To an observer, light sources moving away from them look more reddish as their wavelength seems to stretch out. That means you can get sneaky. Tune the laser to only push back the atoms moving at a certain velocity—a high one—and then do that a bunch of times, and you slow everything down. You make it all colder.
That all works with the antihydrogen that the CERN team makes too. But antihydrogen is a bucket of trouble. “If I go and buy some cesium atoms, I can buy a laser off the shelf that will do this for me,” says Jeffrey Hangst, a physicist and spokesperson for the Antihydrogen Laser Physics Apparatus project, “Alpha,” at CERN. “But because hydrogen is so light, that photon I need is in the vacuum ultraviolet. That light doesn’t propagate through air. It’s completely absorbed.” The laser light isn’t the green of a laser pointer; it’s the ultraviolet of … well, invisible things.
This, in physics terms, sucks. But the researchers don’t really have a choice. “We can’t make antimatter rubidium or cesium,” says Makoto Fujiwara, a research scientist at Triumf, the Canadian particle accelerator center, and head of the Alpha-Canada group. “But to drive hydrogen, you have to have a laser in very short wavelengths and high energy.” This chillaxatron 5000 has to make light at 121 nanometers, very ultraviolet, and shine that light into a bottle of magnetically contained antihydrogen completely in vacuum.
It’s not easy. “Hydrogen is just really hard to laser-cool, because of these bloody ultraviolet lasers,” Hangst says.
The laser has to be precise at a bunch of different jobs. “You have to really precisely control the frequency so we can do the Doppler shift,” says Takamasa Momose, a chemist at the University of British Columbia and one of the laser’s builders. Also, the laser has to put out enough energy in its pulses so the cooling doesn’t take forever.
But it’s not impossible. The team built all that. And when they shot it at antihydrogen, it cooled off just like hydrogen would, already a good sign.
To be clear, it’s not like you can just stick a thermometer into the magnetic trap. You measure this energy differently. Last year, this same team did spectroscopy on their antihydrogen, analyzing it by looking at the spectra of light it emits. Slower-moving atoms emit a narrower spectrum, and when the researchers looked at their post-lasering atoms, that’s exactly what those cold atoms did. They also tested their new results by checking how long it took for their cooled atoms to bounce out of the group and hit the back wall of their container (where, yes, they annihilate). That’s called “time of flight,” and cooler atoms should take longer. They did.
Just as you can’t exactly take their temperature, you can’t point a radar gun at antihydrogen atoms, either. Antihydrogen generally flits around at about 100 meters per second, says Fujiwara, and the ultracool atoms move at just about 10 meters per second. “If you’re fast enough, you could almost catch the atom as it passed by,” he says. (It would annihilate one of your atoms, but you’re tough.)
At this point, it’s reasonable to ask whether this is all worth the trouble. Who needs very slow, very cold antimatter? The answer is, physicists. “Unless something is really screwy, this technique is going to be important, and maybe crucial,” says Clifford Surko, a physicist at UC San Diego who isn’t on the Alpha team. “The way I look at it as an experimentalist is, now you’ve got a whole ’nother bag of tricks, another handle on the antihydrogen atom. That’s really important. It opens up new possibilities.”
Those possibilities involve figuring out whether antimatter really does echo the physics of matter. Take gravity: The equivalence principle in the theory of general relativity says that gravitational interaction should be independent of whether your matter is anti or not. But nobody knows for sure. “We want to know what happens if you have some antihydrogen and you drop it,” Hangst says.
Wouldn’t you? Sure. But this experiment is hard to do, because gravity is actually a wuss. Hot, gassy things don’t fall so much as just bounce around. Antimatter would hit the walls of the machine and annihilate. “Gravity is so bloody weak you may not see anything at all,” Hangst says.
Slow that antihydrogen down to near absolute zero, though, and it starts to act more like a liquid than a gas. Down it blorps, instead of spraying all over. “The first thing you want to know is, does antihydrogen go down? Because there’s a lunatic fringe out there that thinks it goes up—theorists who say there is repulsive gravity between matter and antimatter,” Hangst says. “That would be pretty cool.”
Physicists don’t actually need laser cooling to see if antihydrogen acts like H.G. Wells’ cavorite. That’d be … dramatic. “But if you assume now, as most theorists do, that antihydrogen will fall, then you want to ask, does it really fall in the same way?” Hangst asks. Precisely measuring acceleration due to gravity is the short game for the money here, and laser cooling may well make it feasible.
More spectroscopy is in the works too. That’s hard to do with fast-moving atoms, but slow them down enough and the Alpha team will be able to compare the spectra of antihydrogen and hydrogen. They should be the same to an absurd number of decimal places. But if they’re not? That’d be standard-model-violating new physics.
The team also hopes to look at finer-grained stuff, like the value of the difference between two specific energy levels of hydrogen. This hard-to-measure number, the Lamb shift, ought to be the same for antihydrogen as hydrogen. Again, no one knows if it is. And any of these answers might get back to the bigger question I implied at the top—why is the universe apparently almost entirely matter and not antimatter? Nobody knows that, either, but studying the anti-stuff more closely might help explain it. And eventually the researchers might be able to combine antihydrogen atoms into more stable anti-H2, a hydrogen antimolecule. After that, someday, maybe hydrogen anti-ions, or (if someone invents a way to make other antimatter elements) even bigger and more spectroscopically interesting antimolecules.
This kind of opportunity to actually test some theories doesn’t happen often in experimental physics. But it’s the best part. The particle accelerators at CERN went offline in 2018 for a big refurbishing project. The pandemic delayed their spinning back up. But now the laser lights are coming back on. “There’s nothing we can’t imagine doing that’s been done with hydrogen. That was always the credibility gap—when are you going to prove that you can do what’s being done with hydrogen?” Hangst says. “I think the experts would now agree that we’re there. We have the numbers. We can get the temperatures. We have the reproducibility to study the systematic effects.” He expects the gravity experiments to start in August. The work, once again, will matter.
MAIN IMAGE SOURCE: LIONEL FLUSIN/GETTY IMAGES